1. Field of the Invention
The present invention relates to an optical waveguide fabrication method, which utilizes a phenomenon, whereby the irradiation of an ultraviolet light onto an optical waveguide changes the refractive index of this optical waveguide (hereafter referred to as "photo-induced refractive-index change").
2. Description of the Related Art
Over the years, various methods have been proposed for fabricating optical waveguides. In particular, there is a method for adjusting the refractive index of an optical waveguide, which calls for irradiating a portion of the optical waveguide with ultraviolet light. More specifically, there is a difference in the refractive index of an optical waveguide comprising a germanium (Ge)-doped silica material after it has been irradiated with ultraviolet energy. This is called a photo-induced refractive-index change. This kind of photo-induced refractive-index change phenomenon can be used to readily form a diffraction grating inside an optical waveguide by simply irradiating the optical waveguide with the appropriate ultraviolet energy. For this reason, the above-described method is being practiced in a variety of fields, such as the fabrication of optical filters and wavelength dispersion devices.
A simple explanation of the change in refractive index of an optical waveguide resulting from the irradiation of ultraviolet light is given here. Ultraviolet irradiation of an optical waveguide comprised of a silica material that had not been doped in any way only resulted in a refractive index change of about 0.0001. To fabricate a Bragg diffraction grating with a reflectivity of 99.9% using this optical waveguide required more than 20 mm of the length of the optical waveguide in the direction of the optical axis. Consequently, miniaturization was not possible when manufacturing optical devices containing this optical waveguide. To achieve miniaturization of the optical waveguide, a change of over 0.001 in the refractive index is desired.
Well-known methods for maximizing the change in refractive index of an optical waveguide include high-pressure hydrogen loading (treatment) and a reduction treatment that uses a flame-brush. The high-pressure hydrogen loading is a method, whereby an optical waveguide is maintained for several weeks in a high-pressure hydrogen environment pressurized to over 100 atmospheres, thereby allowing hydrogen to diffuse into the silica material comprising the optical waveguide.
An explanation of the fabrication of a high-reflectivity fiber grating using an optical fiber subjected to a high-pressure hydrogen loading (treatment) is given here. This is described in detail in a paper by Ms. Inai et al (Fall Conference of the Institute of Electronics, Information and Communications Engineers (IEICE), C-208 (1994)). Inai et al conducted an experiment, wherein an optical fiber serving as an optical waveguide was maintained in a 200-atmosphere hydrogen environment at room temperature for 168 hours. This enhanced the sensitivity of the waveguide to ultraviolet light, making it possible to achieve a refractive-index change of around 0.002 via ultraviolet irradiation.
With regard to increasing the refractive index using a flame-brush treatment, Mr. F. Bilodeau et al described this process in detail in "Photosensitization of optical fiber and silicaon-silicon/silica waveguides" (OPTICS LETTERS, Vol. 18, No. 12, P953-P955, Jun. 15, 1993).
The flame-brush treatment is explained using the attached figures. FIG. 7 is a sectional view depicting the general structure of an optical waveguide 30. As shown in FIG. 7, the optical waveguide 30, which comprises a silica material formed on a surface of a substrate 31, is comprised of three parts: a first cladding 33, a core 32 and a second cladding 34. FIG. 8 is a schematic depicting the processes in a conventional optical waveguide fabrication method, which utilizes a flame-brush treatment. The conventional method of fabricating an optical waveguide 30 via a reduction treatment that makes use of a flame comprises a first process (Step S35), whereby a first cladding 33 and core 32 are formed on a substrate 31; a second process (Step S36) for forming an optical waveguide 30 pattern (the part that becomes the core); a third process (Step S37) for forming a second cladding 34; and a fourth process (Step S38) for carrying out a flame-brush treatment and irradiating the optical waveguide with a 248 nm-wavelength krypton-fluoride (KrF) excimer laser to change the refractive index of a portion of the core 32.
In the conventional fabrication method described above, a flame hydrolysis deposition method is used to form the claddings 33, 34 and core 32 in the first process (Step S35) and the third process (Step S37). Flame hydrolysis deposition is a method, whereby a silica powder containing a small amount of Ge is deposited on surface of the substrate 31, after which a flame with a temperature of around 1,500.degree. C. is applied to the silica powder. This flame-based heat treatment causes the silica to melt and become transparent, thus forming the film which becomes the claddings 33, 34 and core 32.
FIG. 9 illustrates the flame-brush treatment process within the fourth process (Step S38). This reduction treatment is a method, whereby a portion of the core 32 is brushed (heated) repeatedly for approximately 20 minutes by a roughly 1,700.degree. C. flame 39 fueled by hydrogen (H.sub.2) containing a slight amount of oxygen (O.sub.2). This reduction treatment heightens sensitivity to ultraviolet light. The flame 39 is provided by way of burner 40. An optical waveguide subjected to a flame-brush treatment can achieve a refractive index change of more than 0.001 upon irradiation with KrF energy.
Fabricating a silica optical waveguide using a high-pressure hydrogen treatment or flame-brush treatment as described above increases the photo-induced refractive-index change. However, the above-described conventional methods posed the following problems. That is, in a silica optical waveguide fabrication method which uses the high-pressure hydrogen treatment, because the hydrogen is under 100 atmospheres or more of pressure, caution is required during operation. Also, as described above, since it is necessary to maintain an optical waveguide in a high-pressure hydrogen environment for a long period of time (for 168 hours in the example given above), this approach makes it impossible to enhance fabrication efficiency.
When it comes to a silica optical waveguide fabrication method which uses the flame-brush treatment, as described above, a heat treatment with a flame temperature of around 1,700.degree. C. is applied to a portion of a substrate. Consequently, when silicon (Si) is used as the substrate 31 material, heating generates considerable heat deformation in the substrate, which in turn can cause cracks in the silica cladding layers and make the core susceptible to birefringence.